Low Poisson's Ratio Stretchable Films Prepared by Hydrogel Temperature Responsiveness

In the research on stretchable films such as wearable display elements or sensors that can be attached to the skin, shrinkage of the film width due to the positive Poisson's ratio has been a problem. Recently, research on auxetic structures with negative Poisson's ratio is being conducted as a solution, but there are limitations in the application because it is 3D-printing structure form which is not a film. Herein, we conducted a study on a film with a low Poisson’s ratio by using cellulose nanocrystal (CNC), poly(N-isopropylacrylamide) (PNIPAm), and hydroxypropyl cellulose (HPC) as fillers and using the alignment of fillers and the shrinkage characteristics of the hydrogels. The composite film was aligned with fillers using directional melt crystallization (DMC) method. DMC is a method for obtaining a porous structure by controlling the three-dimensional temperature gradient. Through the DMC, porous structure with CNC, PNIPAm, and HPC can be obtained, and this is confirmed through SEM. In the case of PNIPAm and HPC, it can be observed through SEM that the existing pores are formed into a re-entrant structure due to hydrogel shrinkage. This re-entrant structure makes low Poisson’s ratio. After shrinking process, stretchable films are manufactured by infiltration of polydimethylsiloxane (PDMS), and polyurethane (PU) to the aligned foam. For CNC film, the Poisson’s ratio is 0.103, and for CNC with PNIPAm film’s Poisson’s ratio is 0.043. This is significantly lower than the PDMS Poisson’s ratio of 0.5, and as the concentration of fillers increases, the Poisson’s ratio tends to decrease. In the case of the CNC and PU film, which is more stretchable than PDMS, the Poisson’s ratio is 0.102 and the strain and young’s modulus values are 500% and 1.28 MPa respectively, which is higher than the CNC with PDMS films. The areal strain ratio of CNC with PNIPAm is 6% and in order to further enhance the effect of the re-entrant structure, it was manufactured by replacing with HPC, and this film of areal strain ratio is 19%. The maximum transmittance of the films in this study reached up to 80% by UV-vis. Through transparency and complementation of physical properties, this film research can be applied to more diverse future material applications.